My app uses several full screen List components, each with several hundred items. In general they perform well. But if I enable grouping I get serious performance issues: they stutter when first scrolled, and data sync operations hang the iPhone 4 for 10 seconds at a time. (Desktop Safari doesn't stutter but it does hang with a beach ball for 2-3 seconds.)

I'd really love to use grouped lists but can't while this is going on. Any idea what it might be? Is this a known issue?